This invention relates generally to switching amplifiers and, in particular, to apparatus and methods for reducing distortion as pulse widths approach zero.
Switching amplifiers often rely on the control of a filtered output voltage by modulating the width of constant-voltage switching outputs. The technique is commonly referred to as pulse-width modulation, or PWM. As these pulse widths approach zero, distortion is introduced in that all known switching devices exhibit propagation delays and therefore are incapable of accurately producing pulses shorter than the aggregate of these delays in a signal chain. Since the resulting distortion is at its worst approaching zero output, xe2x80x9ccrossover distortion,xe2x80x9d a particularly noticeable form of distortion, arises.
One approach used to null these delays has been to modulate a differential pair of outputs, in a bridge configuration, in opposing directions from a median of 50%. This technique is shown in U.S. Pat. Nos. 4,404,526 entitled xe2x80x9cHigh Fidelity Audio Encoder/Amplifierxe2x80x9d and 4,773,096 entitled xe2x80x9cDigital Switching Power Amplifier,xe2x80x9d the entire contents of both of which are incorporated herein by reference. With approximately equal delays extant in both outputs, the distortion is thereby greatly reduced. This approach, however, mandates that both outputs continuously switch, which increases system switching losses.
The present invention resides in a technique for minimizing propagation delay distortion in a switching amplifier without impacting system efficiency. Broadly, the circuitry and accompanying methods of operation are used to detect when the width of an output pulse is approaching, or is less than, the propagation delay of switching devices in the output stage and, if this is the case, the width of the output pulse is artificially adjusted a function of the delay.
More particularly, the invention detects when the amplifier attempts to reproduce sufficiently short duration pulses and, on alternate pulses, adds the width xe2x80x98Nxe2x80x99, being twice the minimum accurate width, to the desired width for output, or subtracts the desired width from the width xe2x80x98Nxe2x80x99 for the output while inverting the polarity of the output.
The net effect is that the desired width is alternately added to, and subtracted from, the width xe2x80x98Nxe2x80x99, thus enforcing an accurately achievable minimum pulse width without imposing error. When applied to a bridged switching output stage, system switching losses remain those of a single switching output.